The invention relates to a modular interferometric recombination device and an electromagnetic beam splitter constituting one component of the device.
The device is particularly intended to be used for accurate interferometric recombination of individual electromagnetic beams from the same source object. The device and the splitter are designed to be used with electromagnetic beams consisting of waves allowing such reconstitution and therefore in particular UHF waves or waves in higher frequency bands including optical frequencies extending from the infrared to the ultraviolet via the visible spectrum.
It is possible to synthesize, in terms of its resolving power, a collector, and in particular a large optical collector, from smaller collectors and in a modular manner, for the purposes of astronomical observation, for example. This technique can be used in particular in fields such as remote Earth sensing and spatial astronomy with very high resolution. It is also used to detect weak objects by coronographic interferometry. One of the main problems associated with producing a device operating as an interferometer in accordance with the above technique is to obtain sufficiently accurate recombination of the individual beams from each of the various collectors, referred to as synthesizing collectors, which synthesize the large collector. It is particularly important for the individual beams to have phase and amplitude characteristics that agree exactly with the requirements of the interferometer in which they are superposed.
This problem is complicated by the fact that, in some applications, it is necessary to divide the energy of the individual beam supplied by a synthesizing collector in order to share it between a plurality of arms of the interferometer, which reduces the average amplitude of the energy available in each of the channels defined by those arms.
A second problem that arises is connected with the fact that interference occurs between individual beams at every point in the cross section of the combined beam obtained when individual beams are superposed coaxially. Because of this, the smallest amplitude difference, relative to the ideal level, affecting the spatial distribution of the individual beams transversely to the propagation direction is reflected in an intensity error depending on the spatial distribution. The stored signal that is obtained by detecting the combined individual beams incorporates the cumulative intensity errors and is therefore incorrect. The error affecting the stored signal can vary greatly because it is affected by the slightest variation in the coaxial relationship between the combined individual beams.
In most cases there is provision for interferometric recombination of individual beams from synthesizing collectors, in xe2x80x9cpupilxe2x80x9d mode, with individual beams that must be perfectly aligned. A semi-transparent plate beam splitter is generally used for this purpose, but the selectivity as a function of wavelength of the reflection and transmission properties of this kind of splitter, which corresponds to chromatic distortion in the visible spectrum, constitutes a serious drawback. The resulting disparities between individual beams induce unacceptable errors that cannot be corrected afterwards and prevent sufficient accuracy of interferometric recombination from being obtained.
The invention therefore proposes an interferometric recombination device made up of at least one and generally a plurality of recombination modules. It also proposes a beam splitter that can be used as a component of the above recombination device.
The device and consequently the module for producing it are adapted to effect optimum interferometric recombination of individual electromagnetic beams and more particularly of beams derived from the same source by a plurality of associated collectors, in particular to synthesize a large optical collector.
In accordance with one feature of the invention, the modular device includes at least one module including:
at least one splitter assembly incorporating a beam splitter provided with interceptor means positioned to split an incident beam from a collector into portions, one of said portions passing through the splitter via an aperture provided for that purpose and the other portion or portions being intercepted by one or more surfaces of the interceptor means that are each adapted either to reflect a beam portion in a different given direction or alternatively to absorb it, and
at least one coupler adapted to effect interferometric recombination of the beams that it receives and which is obtained by localized splicing of waveguides in accordance with an X or Y structure, each of the waveguides receiving a beam from a different collector and at least one of the guides receiving a beam consisting of one of the portions resulting from the splitting effected by a beam splitter.
In one embodiment of the device according to the invention the coupler or each coupler of a module is obtained by localized splicing of at least two monomode optical fibers coupled in an X or Y structure, each of the fibers receiving at an entry end a beam that is transmitted to it from a collector via at least one beam splitter, the beams received by the same coupler coming from different collectors.
In a variant of this embodiment of the device according to the invention a device module includes at least one focusing optic that transmits an incident beam portion coming from a beam splitter toward an entry end of one fiber of a coupler and said optic modulates the amplitude of the beam applied to the entry end of the fiber.
In another variant a module can include at least two beam splitters arranged in cascade.
In one embodiment the device interferometrically recombines optical beams transmitted by collectors in the form of telescopes aimed at the same source of radiation, and includes a plurality of modules as above each associated with at least two collectors in order to effect interferometric recombination of optical beams coming from the collectors.
The invention also provides an electromagnetic beam splitter as defined above.
According to the invention the beam splitter includes interceptor means that are positioned or can be positioned to intercept at least one portion of an incident beam to be split so as to reflect it partly or possibly totally in at least one direction at a reflective surface, the number of directions being fixed by the number of reflective surfaces receiving the beam to be split in the geometrical configuration chosen for the interceptor means, and/or to absorb at least one portion of the beam intercepted by means of an absorbing surface, whilst allowing to pass through it any portion of the incident beam that is not intercepted.
In one embodiment of a splitter according to the invention the interceptor means consist of a plane or curved reflective surface with a circular border that is rendered incomplete by creating an aperture corresponding to the absence of a sector or a segment of the reflective surface so as to split geometrically a beam to be split on whose path said surface is placed to obtain two beams, one reflected by the surface and the other transmitted through the aperture, in proportions corresponding to the respective dimensions of the aperture and the reflective surface on the path of the beam to be split that is split by the splitter.
In another embodiment of a splitter according to the invention the interceptor means consist of a reflective surface with a circular border that is rendered incomplete by creating a central aperture for obtaining the same overall result as that obtained with the embodiment envisaged above.
In another embodiment of a splitter according to the invention the interceptor means consist of a plurality of plane or concave reflective surfaces that are combined and whose contiguous edges delimit lateral faces of a pyramid placed on the path of a beam to be split and whose apex is oriented so that said beam to be split reaches it first, each surface reflecting in a direction specific to it a reflected beam that corresponds to the portion of the beam to be split that it receives.
In a variant embodiment the interceptor means consist of two reflective surfaces combined in a dihedral placed on the path of a beam to be split and whose edge is oriented so that said beam to be split reaches it first, each face reflecting in a direction specific to it a reflected beam that corresponds to the portion of the beam to be split that it receives.